The object of the present invention is a method and device as specified in the independent claims presented below for drying fibre-containing pulp, such as chemical pulp, mechanical pulp, thermomechanical pulp, TMP, deinked pulp or fibre-containing sludge. The invention relates especially to a method in which the wet pulp is first dewatered mechanically in a mechanical water separator such as a drum or screw press, and in which water is then evaporated from the pulp by utilising drying gas.
Today, fibre-containing pulps, such as chemical pulp, are dried largely by means of techniques known from paper machines, possibly in modified form. This means that the wet end of a pulp drying machine, where the pulp web is formed, is required to have properties corresponding to those of the wet end of a paper machine, in order to make the pulp web such that it is able to pass through the entire drying process, all the way through to the baling line, without problem. This type of paper machine technique, which is intended for the manufacture of a more highly refined finished productxe2x80x94paperxe2x80x94is thus unnecessarily sophisticated and also expensive for a pulp product. After all, the sole purpose of a pulp drying line is to remove water from the pulp without damaging the fibres, although it is true that certain properties of pulp, such as its dimensional stability, improve with drying.
The end product obtained from the pulp drying machine, a dried pulp web, always ends up in the pulper, sooner or later, to be mixed with water again. Therefore, basically the only important aspect in drying pulp as well as other similar materials, is to obtain a high dry matter content and yield at the wet end of the pulp drying process. The pulp drying process should be such that the pulp can be dewatered without having to be formed into a pulp web possessing the properties of paper or board.
For this reason it has previously been suggested that the pulp be dried as flakes rather than as a web. In the Flash Dryer flake dryer the flakes dry while mixed with hot gas as they are conveyed pneumatically in conveyor pipes. After drying, the flakes have to be separated in separate cyclones from the carrier gas. A common feature of flake dryers is the short drying time, since water diffuses to the surface more easily from flakes than from inside a compact web. The capacity of conventional Flash Dryer flake dryers is, however, relatively low, typically less than 500 t/day. They also require large duct systems for conveying the pulp-air mixture. This is why, despite the good drying efficiency obtained with the Flash Dryer technique, techniques corresponding to those used in paper machines are generally applied in pulp drying. It has previously also been suggested that a conventional rotary drum dryer be used for drying flaky pulp.
The aim of the present invention is to achieve an improved method and device for drying fibre-containing pulp.
The aim is especially to achieve an easily runnable pulp dryer, that is, a dryer that makes it possible to avoid the difficult web formation stage and achieve the advantages of through-flow drying.
The aim is also to achieve a pulp dryer that has low requirements concerning the pulp material supplied to it, that is, a pulp dryer which allows the use of various mechanical devices for removing water from the pulp before the drying proper.
A further aim is to achieve a pulp dryer with easy start-up and run-down, and which is not susceptible to interruptions.
In order to achieve the aims presented above, the method and device relating to the invention are characterised by what is specified in the characterising parts of the independent claims presented below.
In a typical drying process relating to the invention the pulp to be dried, which has preferably first been dewatered by means of mechanical dewaterers, such as a drum or screw press, to a dry-matter content typically exceeding 45%, and in which pulp the fibres have been separated from one another in a fine shredder or the like, is arranged to pass, according to a preferred embodiment of the invention, through a gap-like drying space limited between a first and a second surface provided with apertures, in which gap or drying space drying gas is blown through the pulp layer in order to evaporate water from it.
In the dryer, the distance between the cylindrical or straight surfaces provided with apertures is preferably within the range of 40-120 mm, typically about 80 mm, which means that a gas-permeable pulp layer having a thickness of approximately 40-120 mm, or approximately 80 mm, can be formed in the annular or otherwise shaped gap formed between them.
The surfaces with apertures can be formed, for example, of perforated plate having e.g. round or gap-like apertures, or of supporting netting onto which is fitted a fine-meshed netting provided with meshes of the desired size.
The surfaces of the pulp dryer that are provided with apertures are preferably cylindrical and fitted concentrically inside each other so that an annular gap is formed between them. One or more means are preferably fitted in the gap, by which means the pulp layer can be conveyed forward in the gap. The pulp to be dried is preferably introduced at one end of the annular gap and the dried pulp is discharged from the other end. The means conveying the pulp layer are preferably wing-like means fixed in a spiral manner on the inner cylindrical surface. The inner cylindrical surface is preferably arranged to rotate around its axis, whereby the spirally affixed wing-like surfaces, which preferably extend from the first surface almost to the second surface, convey the pulp layer forward as they rotate. The means may obviously also be fixed to the second surface. If so desired, this second surface may rotate.
The diameter of the inner cylindrical surface or of the inner tube may, for example, be about 2-5 m. The length of the cylinders may, for example, be about 15-40 m.
The drying gas, typically drying air, is blown through the apertures in the first surface mainly perpendicularly towards the pulp layer travelling in the drying space between the surfaces, and is discharged from the said pulp layer through the apertures in the second surface. The first and/or second surface provided with apertures is preferably arranged to support the pulp layer.
If the drying air is always blown towards the same side of the pulp layer, e.g. always through the inner cylindrical surface towards the outer cylindrical surface, the pulp layer will dry more rapidly on the side that is towards the surface of the inner cylinder. In some cases this may not be desirable.
In such a case, the drying gas may be blown through the pulp layer alternately in one direction and its opposite direction. This may be achieved, for example, by dividing the exhaust gas collection hood covering the outer cylinder into two parts, in one of which there is underpressure and in the other overpressure compared with the internal pressure in the inner cylinder. In this way, the drying gas can be made to travel radially in different directions at different points in the gap-like drying space, that is, the drying gas is alternately blown inwards towards the inner cylinder and sucked outwards from the inner cylinder. The pulp to be dried, which is arranged to travel forward, for example, along a spiral path around the inner cylinder, will thus alternately pass the point where the drying gas travels inwards from the outside and the point where the drying gas travels in the opposite direction. In this way the pulp layer can also be dried evenly through the web.
In the dryer, the pulp layer is dried with drying gas at a temperature of about 100-300xc2x0 C. from a dry matter content of about 30-60% to a dry matter content of over 80%, typically about 85-90%. Drying may take place in one or more at least partly separate successive stages, which means that the temperature may differ at the different stages.
The pulp to be dried is preferably formed into a mat-like air-permeable pulp layer of even thickness over the casing of the inner cylindrical surface. The second casing of the outer cylindrical surface supports the pulp layer on its other side while the wing-like means push the pulp layer along the casing of the inner surface towards the outlet. The density of the pulp layer is advantageously about 0.2-0.5 t/m3, preferably about 0.3 t/m3.
The dryer relating to the invention, which is provided with cylindrical surfaces, may be a modified screw conveyor comprising
a perforated rotating inner tube which forms a first cylindrical surface,
a perforated outer casing which forms a second cylindrical surface,
means fitted in a spiral manner, xe2x80x9ca screwxe2x80x9d, for pushing the pulp layer forward in the annular space between the inner tube and the outer casing,
a closed cover or hood covering the outer casing, and
means for feeding drying gas from the perforated inner tube through the pulp layer in the gap-like space, and further through the perforated outer casing, into the closed hood or cover.
In a screw conveyor-type solution, the structure enables the use of quite considerable pressure differences across the web, even in the case of weak-fibre pulps, such as deinked pulps. The pressure differences may be of the order of about 500-1000 Pa, or even more. The technique is simple. The dryer is easy to start up and run down. It is not susceptible to interruptions.
There are no special requirements as regards the pulp fed to the dryer relating to the invention. It is, however, advantageous to separate water from the pulp mechanically before drying proper, so that its dry matter content will be 30-60%, typically about 5%, which means that it will be unnecessary to use a large amount of thermal energy to remove the said water. Water can be separated from the wet pulp by means of a wide variety of mechanical devices, such as drum or screw presses or precipitators.
In a dryer relating to the invention, the drying proper performed by means of a hot gas can be arranged to take place in two or more successive stages connected in series. This allows the necessary number of shorter dryers similar to a screw conveyor, or other types of dryers relating to the present invention to be connected in succession on one drying line. The dryers are connected so that the feed inlet of the second stage is connected to the discharge outlet of the first stage, etc. This means that the dryer of the first stage can advantageously be fitted on a higher level than the dryer of the second stage, in which case the pulp dried at the first stage transfers easily to the second stage on a lower level. The dry matter content of the pulp is increased stage by stage. The drying may take place, for example, in three stages, in which case the pulp is dried from a dry matter content of 35% to a dry matter content of 50% at the first stage, and further to a dry matter content of 70% at the second stage, and further to a dry matter content of 95% at the third stage.
Phased drying can also be achieved in a single dryer by dividing the dryer into several successive drying zones or segments.
In a dryer divided into different drying zones or segments, or in several successive dryers, the drying process can be regulated by arranging so that at least some of the separate parts, segments or zones of the dryer have their own return air systems. In this way, considerably higher drying gasxe2x80x94typically drying airxe2x80x94temperatures can be applied at the beginning of drying than at the end of drying. At the beginning, when there is still a lot of free water in the pulp, the temperature of the fibres will not rise too high, and thus the application of high temperatures will not impair the quality of the pulp. At the end of drying, when the water is mainly bound water, lower temperatures can be applied. The temperature of the pulp to be dried can also be affected by adjusting the humidity of the return air. The temperature of the pulp always settles at a temperature corresponding to the wet bulb temperature of the return air.
By connecting several dryers adjacent to one another it is possible to achieve a high capacity, good flexibility of capacity, and also the possibility of servicing without stopping the entire drying line. Furthermore, the dryer can be arranged to have an automatic cleaning stage for the surfaces with apertures, or it can be provided with brushes for self-cleaning.
The capacity of the dryer is affected e.g. by air flow variables, the thickness of the pulp layer, the speed at which the pulp advances in the gap-like drying space. The speed of advance of the screw is, for example, about 0.2 m/s. The speed of advance of the pulp layer is dependent on the advance of the wing-like means, that is, the screw conveyor, and on the speed of rotation.
In the dryer relating to the invention, the advantages of a screw conveyor and a drum dryer are combined in an advantageous manner. The drying takes place by a through-flow technique, by means of which high evaporation per square meter is achieved. In the solution relating to the invention, however, the difficult web formation stage that is normally required in conventional pulp drying in order to achieve uniform drying is not required.
Since in the solution relating to the invention the pulp to be dried does not have to be formed into a web, this solution can also be used for drying considerably weaker pulp grades than before, such as deinked pulps or new types of pulps such as maize or cane pulps.